nebula filter

A nebula filter is a special type of filter made so as to enhance the contrast
of many nebulae and thereby enable the observer
to see nebulae that are otherwise not very visible in light-polluted locations;
they can also enhance the visibility of some nebulae even at very dark observing
sites. Nebula filters are manufactured with vacuum-deposition technology
to create coatings that transmit very specific wavelength regions and block
the rest of the visible spectrum to increase contrast for certain nebulae.

Narrowband nebula filters, commonly called ultra
high contrast (UHC) filters, pass the hydrogen-alpha,
hydrogen-beta, and oxygen-III spectral wavelengths and block the rest of
the visible spectrum; O-III filters pass only the oxygen-III
wavelength. (Since stars are broadband emitters these filters will not help
in viewing stars, star clusters, or galaxies – indeed, they will just
make those objects more dim.)

A third type of filter called an H-beta blocks all light
but the hydrogen-beta wavelength and is only useful for a few specific nebulae,
such as the Horsehead Nebula, California
Nebula, and Cocoon Nebula, that
strongly emit in this line. Finally, there is the LPR (light pollution
reduction) filter – a broadband filter intended to mitigate
the effect of common light pollution sources such as street lights.